1. Field
This application relates to communication networks and, more particularly, to a method and apparatus for direct frame switching using frame contained destination information.
2. Description of the Related Art
Data communication networks may include various computers, servers, hubs, switches, nodes, routers, proxies, and other devices coupled to and configured to pass data to one another. These devices will be referred to herein as “network elements.” Data is communicated through the data communication network by passing protocol data units, such as frames, packets, cells or segments, between the network elements by utilizing one or more communication links. A particular protocol data unit may be handled by multiple network elements and cross multiple communication links as it travels between its source and its destination over the network.
The various network elements on the communication network communicate with each other using predefined sets of rules, referred to herein as protocols. Different protocols are used to govern different aspects of the communication, such as how signals should be formed for transmission between network elements, various aspects of what the protocol data units should look like, and how protocol data units should be handled or routed through the network by the network elements.
A communication network may be viewed using what is commonly referred to as the seven layer Open Systems Interconnect (OSI) network model. In this model, there are seven layers of abstraction, ranging from physical mediums that actually handle the traffic at Layer 1 to applications that interact with each other and with users at Layer 7. The second layer, referred to in the OSI network model as the “link layer,” will be of particular interest herein. Network elements that operate at the link layer are not network aware—that is they don't know about the overall network topology and hence do not make routing decisions as that term is commonly used. Rather, layer 2 network elements are configured to match an input protocol data unit with an output port and transmit the protocol data unit over the link attached to the identified output port. Protocol data units on the data link layer are commonly referred to as “frames.”
There are two basic types of network elements that operate at the link layer: hubs and switches. Hubs transmit everything they receive over an interface on all other interfaces (ports). Switches, by contrast, are designed to output traffic to a particular port depending on where the traffic is to be sent. Generally, a switch operates by examining the content of a frame to determine the destination Media Access Control (MAC) address of a frame. Using the destination MAC address it then performs a table lookup to determine the interface through which the destination address may be reached, and outputs the frame on that interface.
In a conventional layer 2 switch using common layer 2 forwarding techniques, the switch learns the MAC addresses of attached network elements and creates a table of destination MAC addresses and output port IDs so that the switch can forward frames to the proper destination ports. This table is constructed dynamically as the switch learns the MAC addresses of attached devices from the source MAC address fields of received frames. A received frame is forwarded to an appropriate port based on the frame's destination MAC address if that destination MAC address is contained in the forwarding table. If the destination MAC address is not in the forwarding table, the frame is generally sent to all ports (except the receiving port).
This leads to two drawbacks. First, proper frame forwarding can only take place once the switch's forwarding table has been populated. Before that, frames are sent to all ports thus generating excess network traffic. Second, the layer 2 switch is required to search its forwarding tables to determine whether it knows the destination port and to identify the correct destination port for each received frame. As larger switches are created and configured to connect to larger numbers of network elements, the large number of destination MAC addresses increases the size of the switch forwarding table, which generally translates into increased processing time to locate a destination port associated with a destination MAC address. At the same time, overall bandwidth on communication links is increasing, thus increasing the frequency at which frames can be expected to arrive a switch and, hence, reducing the amount of time available to the switch to process each frame. Accordingly, it would be advantageous to enable switches to handle frames using frame contained destination information.